A numerically controlled lathe (hereinafter referred to as the NC lathe) including a guide bush is known, and the guide bush is configured to rotatably support a workpiece in the vicinity of a processing position where the workpiece is processed by a tool attached to a tool post. The guide bush supports the workpiece which is being processed in the vicinity of the processing position to suppress swing and deflection generated in a processing target portion owing to rotation, whereby the workpiece can be processed with high accuracy.
FIG. 8 is a plan view showing a conventional example of the NC lathe provided with the guide bush.
On a bed 201 of an NC lathe 200A, a head stock 210 which roatably supports a spindle 220 is provided. The head stock 210 can move forwards and backwards in a Z-axis direction which is the same direction as that of a spindle axis C extending through the center of the rotation of the spindle 220. The rotation center of the spindle 220 is provided with a through hole (not shown). Into this through hole, A long-bar-like workpiece W is inserted. The workpiece W is grasped by a chuck (not shown) provided at the tip of the spindle 220.
On both sides of the spindle axis C, a first tool post 230 and a second tool post 250 are disposed so as to face each other.
On one side (the downside in FIG. 8) of the head stock 210, a guide rail 231 is disposed on the bed 201 in a Z1-axis direction parallel to a Z-axis, and on this rail, a tool post main body 233 of the first tool post 230 is mounted. The tool post main body 233 can move forwards and backwards in the Z1-axis direction while being guided along the guide rail 231, by driving of a driving member (not shown). The front surface (on the left side in FIG. 8) of the tool post main body 233 is provided with a guide rail 235 in an X1-axis direction crossing the Z-axis at right angles, and on this rail, a slider 241 having a tool attaching portion 240 is mounted. The slider 241 can move forwards and backwards in the X1-axis direction while being guided along the guide rail 235, by driving of a driving member (not shown). To the tool attaching portion 240, a plurality of comb-teeth-like tools T1 are attached via a holder 242 in a direction crossing the X1-axis and the Z1-axis at right angles. The holder 242 is movable in an axial direction (hereinafter referred to as the Y1-axis) crossing the X1-axis and the Z1-axis at right angles, and by a combination of the movement of the tool post main body 233 in the Z1-axis direction, the movement of the slider 241 in the X1-axis direction and the movement of the holder 242 in the Y1-axis direction, the tool T1 is indexed and positioned with respect to the workpiece W.
On the other side (the upside in FIG. 8) of the head stock 210, a guide rail 251 is disposed on the bed 201 in a Z2-axis direction parallel to the Z-axis, and on this rail, a tool post main body 253 of the second tool post 250 is mounted. The tool post main body 253 can move forwards and backwards in the Z2-axis direction while being guided along the guide rail 251, by driving of a driving member (not shown). The front surface (on the left side in FIG. 8) of the tool post main body 253 is provided with a guide rail 255 in an X2-axis direction crossing the Z-axis at right angles, and on this rail, a slider 261 having a tool attaching portion 260 is mounted. The slider 261 can move forwards and backwards in the X2-axis direction while being guided along the guide rail 255, by driving of a driving member (not shown). To the tool attaching portion 260, a plurality of comb-teeth-like tools. T2 are attached via a holder 262 in a direction crossing the X2-axis and the Z2-axis at right angles. The holder 262 is movable in an axial direction (hereinafter referred to as the Y2-axis) crossing the X2-axis and the Z2-axis at right angles, and by a combination of the movement of the tool post main body 253 in the Z2-axis direction, the movement of the slider 261 in the X2-axis direction and the movement of the holder 262 in the Y2-axis direction, the tool T2 is indexed and positioned with respect to the workpiece W.
In front of the head stock 210, a guide bush 270 is provided between the first tool post 230 and the second tool post 250. This guide bush 270 is disposed so as to support the workpiece W along the spindle axis C, and fixed onto the bed 201. The guide bush 270 includes such a stational type that the inner peripheral surface of a guide hole into which the workpiece W is inserted is constituted as a plain bearing surface with respect to the rotating workpiece W, and such a rotary type that the inner peripheral surface of the guide hole rotates together with the workpiece W while substantially coming in contact with the outer peripheral surface of the workpiece W. As the guide bush 270, either the above-mentioned stational or rotary type may be selected as long as the workpiece W can move in the direction of the spindle axis C.
To process the workpiece W by the NC lathe 200 having the above constitution, the workpiece W is grasped by the chuck provided at the tip of the spindle 220, and in this state, the head stock 210 is moved toward the guide bush 270 so that one end of the workpiece W is protruded as much as a predetermined length from an outlet 271 of the guide bush 270.
Moreover, the spindle 220 is moved in the Z-axis direction while being rotated, and the spindle 220 is moved in the Z-axis direction to forward the workpiece W from the guide bush 270 to a processing position. Thus, the processing given to one product can proceed, and the relative movements of the tools T1, T2 and the workpiece W in the Z-axis direction can be caused. Then, a large number of products can continuously be formed from one long workpiece W.
According to the NC lathe 200 including the guide bush 270, the workpiece W is processed in the vicinity of the guide bush 270 which supports the workpiece W, and hence there are advantages that the influences of the deflection of the workpiece W itself and deflection generated by the abutment of the tools T1, T2 on the workpiece W can be decreased and that the workpiece W can be processed with high accuracy.
However, when the processing dimension (a length to be processed in the Z-axis direction) of the workpiece W by the tools T1, T2 increases, the length of the workpiece W to be protruded from the guide bush 270 also increases. In consequence, the deflection of the workpiece W and swing generated during the rotation of the spindle 220 increase, with the result that the processing accuracy of the workpiece W deteriorates.
To solve the above problem, in an automatic lathe disclosed in Patent Document 1 by the present applicant, an auxiliary guide is provided. This auxiliary guide can move between a processing position where an extending portion of the workpiece forwarded from the guide bush is processed and a standby position which is distant from this processing position. When the auxiliary guide is at the processing position, the extending portion of the workpiece can be supported. This auxiliary guide is provided so as to face the guide bush. When the workpiece is supported by the guide bush and the auxiliary guide, it is possible to suppress the influences of the deflection of the workpiece, the swing generated during the rotation of the spindle and the like, whereby a hole can correctly be made in the workpiece along the rotation axis thereof by use of a hole making tool such as a drill.